Post on 11-Jul-2015
transcript
Thin film Pb photocathodes prepared with the Thin film Pb photocathodes prepared with the cathodic arc cathodic arc
R. Nietubyć, J. Sekutowicz, P. Kneisel, J Smedley
The Goal
The goal is to build a Nb injector with theThe goal is to build a Nb injector with the superconducting superconducting cathode made of lead for use in CW or near-CW operated high cathode made of lead for use in CW or near-CW operated high average current acceleratorsaverage current accelerators
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
Capabilities I, promissing properties of Pb
Superconductor of 1 st type
Critical H 8 mT@ 1.3 GHz at 2 K
Critical T 7.9 K
Work function 3.95 eV
Pb can be used as a superconducting photocathode and deliver 105 1 nC bunches per second when flashed with 4μJ pulses of 213 nm light
The goal was to build an ultra high vacuum cathodic arc deposition facility dedicated to niobium coating with lead and to establish the Pb/Nb coating technology. This effort should lead us to the repetitive and reproducible lead deposition in the photo-injector cavities.
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
Calculations of J. Smedley et al
The phenomena critical for superconductivity
• ohmic centres (defects, impurities [oxides])
• electron emission (roughness)
are closely related to cavity wall morphology
Capabilities II, preparation method
Deposited material arrives in a form of highly energetic ions emitted from the cathodic spot, which can be additionally accelerated by external electric field
Interaction with substrate and already deposited layer:
•sub-plantation
•enhanced diffusion in outer layer
•permanent exceed of material
•gradual cooling and condensation
50x10-3
40
30
20
10
0
ions
[1/Å
]
50403020100depth [Å]
primaries
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
100 eV
Capabilities III, cathodic arc experience at SINS
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
A laboratory has been established in the Plasma Physics Department at SINS in 2004. It participating in thin film SRF cavity programme performed in the frame of CARE. It resulted in an established procedure of three-cell Tesla-like Cu cavity coating with 2 µm Nb layer. A disadvantage was poor adhesion (< 80 bar HPWR)
3-cell cavities structure was coated For samples Nb/sapphire RRR=43
Proof of principle experiment
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
Various Pb samples were prepared:
Lead layer has been deposited onto Nb in the simplest geometrical arrangement. Substrate was placed in front of arcing cathode.
Next it was cleaned with KrF 10 ns pulses 248 nm 0.3 mJ/mm laser pulses
QE was measured
magnetron sputteredelectro-platedvacuum depositedarc depositedbulk lead
Arc deposited layer showed the highest QE
Lead QE vs Photon energy
1.0E-04
1.0E-03
1.0E-02
4.00 4.50 5.00 5.50 6.00 6.50 7.00
Photon energy (eV)
QE
Theory
Measurement
Experimental results confirm the calculations
Second approach - cavities
Those trial depositions showed the capability of cavity treatment. In particular to match cleanliness requirements and to solve mechanical problems
Pbpure
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
Regular devlopement of the method
Optimization of deposition system• transmission and deposition rate• micro-droplets filtering• temperature control• cleanliness and vacuum
Photocathodes preparation• deposition processes • after deposition treatment
• chemistry• laser flashing
Measurements• surface diagnostics• QE
(M18)Lead deposition on half cells and 1.6 cell cavitiesM10.4.2
(M12) Lead deposition on samples for photocathode developmentM10.4.1
Milestones
(Report, M36)Cold test results for the test cavities with and without the deposited lead photo cathodeD.10.4.3
(Report, M12)QE data for Pb/Nb deposited photo cathode samplesD.10.4.1
Deliverables
WP 10.4
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
Deposition system with knee-shaped microdroplets filter
polarisation = -110 V base pressure <10-7 mbar
arc current = 25 Acoil current = 120 Aarc voltage = 17-18 Vion current = 2.5 mAdeposition rate ≈ 0.5 nm/s deposition time < 80 minwall temperature <32 °C
2.0x10-12
1.5
1.0
0.5
0.0
curre
nt
50403020100mass [at. units]
Rest gases
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
Optimisation of depsotion, microdroplets removal
Microdroplets removal – knee-shaped filter
dryBent1.6 cellpoly60008
dryBent0.5 cellmono27007
dryBent0.5 cellmono27006
dryBent1.6 cellpoly27005
dryBent1.6 cellpoly27004
oilStraight1.6 cellmono18003
oilStraight1.6 cellpoly18002
oilStraight1.6 cellpoly18001
PumpSetupDistanceNb typeTime [s]No
SEM pictures of as-delivered sample No 1 (left) and sample No 5 (right). Note different scale.
Location of the heater with lead stub for plasma formation
M.10.4.1 Lead deposition on samples for photocathode development
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
Micro-droplets were efficiently removed. Unfortunately in cost of very low transmission
Modeling a filter shape:The best was case a when applied:200 A which gave
12 - 16 mT
Verification with sample plugs
Optimisation of depsotion process, QE measurementsD. 10.4.1 QE data for Pb/Nb deposited photo cathode samples
QE with lamp sourceBefore Cleaning
-0,00005
0
0,00005
0,0001
0,00015
0,0002
0,00025
4 4,5 5 5,5 6 6,5 7
Photon Energy (eV)
QE
Laser: 213 nm 1 min 25 Hz 0.2 mJ/mm2 per pulse
QE with lamp source0.21 mJ/sq mm Cleaning
0
0,0005
0,001
0,0015
0,002
0,0025
4 4,5 5 5,5 6 6,5 7
Photon Energy (eV)
QE
1 MV/m
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
Pb
Nb
QE=2.110-4 QE=2.010-3
Laser cleaning improved the QE, mostly by oxide removal. Unfortunately, it caused a lead re-crystallisation. QE measure for cleaned sample. QE would be at least twice higher if the coverage was complete.
(i.e. 20 times higher then for Nb)
(i.e. still worse than measured in the proof of principle
experiment
How to avoid a layer destruction?
• by making the layer thicker.
• by making the laser cleaning smoother
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
Increased transmission to enable thicker filmsTo improve the coating efficiency, the system has been modified by replacing the rectangular knee by the 30º bent tube. That solution ‑enabled to increase 3 times the ion current saving the lead flux free of macro-particles. Chosen angle provides the minimal bend angle for which the lead droplets, which all move rectilinearly, cannot reach the target
30°coil current = 70 Aion current = 8 up to 14 mAdeposition time < 80 min
no micro-droplets
Optimisation of depsotion process, transmission
Optimisation, final resuts for plug samples
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
Gentle laser treatment : 190 nm, 30 min, 300 Hz 0.01 mJ/mm2 per pulse
as compared to:213 nm, 1 min, 25 Hz, 0.2 mJ/mm2 per pulse
QE with lamp sourceCenter, No Cleaning
-0,0001
0
0,0001
0,0002
0,0003
0,0004
0,0005
4 4,5 5 5,5 6 6,5 7
Photon Energy (eV)
QE
1 MV/m
QE with lamp sourceCleaned, 0.75 mJ/sq mm
0
0,0005
0,001
0,0015
0,002
0,0025
0,003
0,0035
4 4,5 5 5,5 6 6,5 7
Photon Energy (eV)
QE
1 MV/m
QE=3.310-3
Improved deposition geometry and gentle laser cleaning procedure enabled the layer to survive in much better shape. As a result the highest QE was reached. It is however still lower than that measured in the proof of principle experiment. Still a thicker layer is needed.
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0keV
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
cps/eV
Pb Pb Pb C O
Carbon and oxygen contamination
As deposited Pb 0.904, C 0.063 O 0.033
Cleaned Pb 0.930, C 0.045. O 0.025
Pb deposition onto cavities back wall of, a setup
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
Mask:
a stainless steel tube (1) with a bellow (2) and niobium cap (3)
1 2 3
1
23
DESY cavity 1, March 2010
1.E+09
1.E+10
1.E+11
0 10 20 30 40Epeak [MV/m]
Q Cold RF test performed for the coated cavities showed quality factors (Q) close to 3109 at gradients on the
cathodes of 29 MeVm-1 and 39 MeVm-1.
M.10.4.2 Lead deposition on half cells and 1.6 cell cavities
effective deposition time = 80 min (16×5 min arc + 30 min cooling)base pressure < 2⋅10-7 mbarcoil current = 75 Aion current = 10 mAdeposition time = 80 minwall temperature < 34 °Cthickness < 200 nm (roughly from EDS measurements for samples)
Typical BCP acid treatment and HPWR were applied. The photocathode was chielded with the mask
That promissing test has been interrupted by a helium leakage and could not been continued after it was mend. The cavity was sent to Świerk to prepare another Pb layer. Typical BCP acids mixture is capable to remove the old layer
A final test before coating the cavity to be used in Hobicat
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
HZB cavity for Hobicat, May 2010
Back wall consist of three single crystalline domains.
A heluim tank is assembled.
Beam quality test are planned for this cavity in the end of 2010
Exactly the same deposition procedure was repeated for this cavity
A uniform spot of 1 cm in diameter was obtained. Next to a deposition the cavity was filled with N 2 and sent to JLab
20 days later, when it was opened
Film got orangeAfter HPWR it got blackNext it disappeared during the standard treatment
Lead nitride Pb2N3 or Pb3N4
In contrary to PbO does not passivate the surface but penetrate in depth
As the layer was successfully deposited and spoilt only due to the reaction with N 2 We considered that the experiment is
worth of severe work and schedule rearrangement in order to try again. The cavity was sent to Świerk and coated again.
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
DESY cavity 2, last week
coil current = 75 A, enhanced coil wrapping around the back wallion current = 11 - 14 mAdeposition time = 80 min
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
4,9589610×60
4,961445×120
4,96714120
4,9642360
4,9644915
4.958993
;lattice constant
Dep. time
8
6
4
2
0
log(
coun
ts)
12010080604020
2θ [deg]
log
(cou
nts)
10080604020
2θ [deg]
iiolon703 iiolon711 iiolon715 iiolon719 iiolon723 iiolon727 iiolon731 iiolon735
Lattice constant does not depends on thickness
Orientation distribution does not change neither
Amorphous or nanocrystalline phase in the interface
Observable amount of PbO appears in roughly 10 hours of the exposition to air
XRD studies of Pb film growth on sapphire and Nb
Reflectometry, XRD and EXAFS measurements have been performed for film of various thicknesses. Experimental results will be interpreted by ab initio calculations. Only rough results here
time
2h
18h
PbO
6
4
2
(cou
nts)
45403530252 θ [deg]
Lattice constant vs thickness
Two lead phases
oxidation
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
Conclusions
We gained:
Reproducible processes (4 times for 1,6-cell cavities)
Micro-droplets removal
Optimised laser cleaning procedure
Reasonable QE
We will continue with:
Beam tests of Pb/Nb injector at Hobicat at HZB (end of 2010)
Contaminations removal
Fatting the film
System reconstruction in order to shorten the plasma channel
M.10.4.1 Lead deposition on samples for photocathode development.10.4.1D / QE data for Pb Nb deposited photo cathode samples
M. 10.4.2 - 1.6- Lead deposition on the half cell and cell cavities.D 10.4.3 Cold test results for the test cavities with and without the deposited lead photo cathode
Robert Nietubyć, SINS Świerk, Poland4th International Workshop on Thin Films and New Ideas for pushing the limits of RF Superconductivity, Legnaro 2010
Thanks for the attention
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
4 5 6 7
QEPhoton Energy (eV)
After Laser Cleaning
Before Laser Cleaning
1.E+09
1.E+10
1.E+11
0 10 20 30 40Epeak [MV/m]
Q
Q
QE
together